1. Field of the Disclosure
The present disclosure relates to a handover management method and a base station using the same method.
2. Description of Related Art
For wireless communication systems such as the long-term evolution (LTE) communication system, 802.16, Wideband Code Division Multiple Access (W-CDMA), and so forth, a base station (BS) would initiate a handover procedure when a user equipment (UE) has been detected to move out of a coverage area in order to ensure that the wireless service provided for the UE is not disrupted. FIG. 1 illustrates an example of a typical handover procedure of LTE. It is assumed that UE 105 is moving into the coverage area 104 of the BS 103 from the coverage area 102 of the BS 101. In step S106, when the UE 105 is in the coverage area 102 of the BS 101, the BS 101 would configure handover parameters and signal UE 105 to perform a signal measurement. In step S107, when the UE 105 has measured that the received signal strength from the BS 103 is larger than the signal strength received from the BS 101 (e.g., A3 event threshold has been exceeded) as the UE 105 is in the overlapping area between coverage area 104 of BS 103 and coverage area 102 of BS 101, the UE 105 would report a notification (e.g., the A3 event has been triggered) to the BS 101. In step S108, the BS 101 and the BS 103 would perform the handover procedure in response to receiving the notification. After the handover procedure has been performed in step S109, the UE 105 would connect to the BS 103.
On the other hand, the technology of the smart antenna is developing rapidly in the application of the wireless communication in the last few years. Because the technology of smart antenna would be able to detect current network environments as well as user behaviors and would also alter the directivity and radiation pattern of an antenna by using the technology of the beam-forming to elevate the resource efficiency of a spectrum and quality of signals, the technology of the smart antenna could be considered as one of the leading technologies for the next generation (or 5th generation) of communication standard. However, the technology of smart antenna has not been implemented in the current LTE communication systems as lots of mechanisms are designed based on omnidirectional antenna. One of the mechanisms for optimization of handover behavior is call as Mobility Robustness Optimization (MRO) which is proposed by Third Generation Partnership Project (3PP) and is an important function of Self Organization Network (SON) of a small-cell BS. MRO's basic concept is to perform progressive modifications by using history data of users' handover behaviors, so as to reduce situations of abnormal handovers (e.g., too-early handover or too-late handover).
Taking FIG. 1 as an example of a typical MRO procedure. The BS 101 would analyze records of the past handover/abnormal handover such as handover result, timestamp, etc. Next, the BS 101 may adjust handover parameters according the analyzed records. Subsequently, the BS 101 would notify the UE 105 the updated handover parameters so that the UE 105 could perform a handover based on updated handover parameters with higher probability of a successful handover. However, a traditional progressive modification of MRO is only utilized for an omnidirectional antenna with fixed transmission signals and thus would be difficult to be utilized for a smart antenna with varying transmission signals.
FIG. 2 is an example that illustrates a coverage area of two smart antennas with different beam patterns. When BS 201 with smart antenna switches the beam patterns, for example, beam pattern #9(206) to beam pattern #2(207), the distribution of transmission signals of whole wireless network may be thus switched for the BS 201. The previous recorded handover history data (e.g., handover from beam pattern #9(206) of the BS 201 to beam pattern #9(209) of the BS 202) and the previous configured system parameters may not suitable for the current network environment, and therefore system parameters would need to be re-configured again. Furthermore, if a BS with a smart antenna switches the beam patterns too often, the mechanism of MRO may be difficult to obtain optimal handover parameters. If the UEs or the BSs are configured with unsuitable handover parameters, situations such as abnormal handovers, handover failures, or service interruptions may occur. Consequently, the quality of system service would be decreased.